The present invention relates to a method for producing an electrophotographic recording material of the type composed of selenium, selenium compounds, or alloys with selenium applied to a conductive carrier if required via an intermediate layer.
Electrophotographic methods and apparatus to practice such methods are widely used in the reproduction art. They utilize the property of such photoconductive material whereby its electrical resistance changes when it is exposed to activating radiation.
By electrically charging a photoconductive layer and then exposing it to such activating radiation in a pattern determined by an optical image, it is possible to produce thereon a latent electrical charge image which corresponds to the optical image. At the exposed points there occurs such an increase in the conductivity of the photoconductive layer that a part or substantially all of the electrical charge can flow off through the conductive carrier while at the unexposed points the electrical charge remains substantially unchanged. More precisely, the quantity of charge flowing off is greater at the exposed points than at the unexposed points. The latent image can be made visible with a picture powder, a so-called toner, and the resulting toner image can finally be transferred to paper or some other medium, if this should be required.
Organic as well as inorganic substances are used as the electrophotographically active substances. Among them, selenium, alloys with selenium and selenium compounds have gained particular importance.
The conductive carriers preferably are made of aluminum or aluminum alloys.
In order to obtain an electrophotographic recording material of high quality it is required, to vapor-deposit the photoconductive layer on the carrier at a temperature above the glass transformation temperature. This is the opinion of experts in the art (Bixby, U.S. Pat. No. 2,970,906; Felty, New Photoconductors for Xerography, Reprographie II, II. Internationaler Kongress, Cologne 1967, Verlag Dr. O. Helwich, Darmstadt und Wien, 1969; John H. Dessauer and Harold E. Clark, Xerography and Related Processes, The Focal Press, London and New York; R. M. Schaffert, Electrophotography, The Focal Press, London and New York) and, according to the generally known and accepted state of the art. This produces better wetting of the substrate surface, better layer structure, and better surface quality of the photoconductive layer due to less flaws. At the same time a reduction in the potential and an increase in the electrical resistance of the photoconductor can be noted. It should be noted that the "transformation temperature" of glass is defined as being that temperature at which glass has a viscosity of 10.sup.13.4 poises.
In the application of the above-mentioned teaching the vapor-deposition temperatures for pure selenium, for example, should be above about 50.degree. C. For photoconductive layers which contain arsenic in addition to selenium and have higher glass transformation temperatures, the vapor-deposition temperatures must be selected correspondingly higher according to this teaching. With a content of 1 percent by weight arsenic, this temperature should be higher than 55 to 60.degree. C, with 2 percent by weight arsenic this temperature should be higher than 60.degree. to 65.degree. C, with 38.7 percent by weight arsenic (As.sub.2 Se.sub.3), the temperature should be higher than 180.degree. C.
In such methods, where the vapor-deposition temperature lies above the glass transformation temperature, there is the drawback that a substantial amount of apparatus is required to transfer the required heat to the substrate, heat the substrate to the required temperature, and control and regulate the maintaining of this temperature, all during vacuum deposition.
At high vapor-deposition temperatures there appears the further drawback that only part of the material used as the photoconductor adheres to the surfaces of the conductive carrier intended to be coated; the remainder of the vapor-deposited photoconductive material is deposited on the walls of the apparatus. This results in rapid soiling of the apparatus thereby requiring additional cleansing steps. Also, a not insignificant loss of valuable photoconductor material occurs, and, after it is removed from the apparatus it no longer meets the high purity requirements. Due to this phenomenon, called the "re-evaporation rate", the consumption of photoconductive material is much higher than it should be according to the quantity actually deposited. This is particularly noticeable, for example, with arsenic selenide (As.sub.2 Se.sub.3) which has a rather high glass transformation temperature, above 180.degree. C, and when this material is used a substantially larger amount of photoconductor material is always consumed.